Multi-stage transmission

ABSTRACT

Disclosed is a constantly-meshing-type multi-stage transmission which can be operated with small forces, does not need a clutch for gear shift, has no loss of changeover time at the time of gear shift, and in which a loss of the driving force is prevented. Driven gears n 1 , n 2 , . . . n 5  are supported on a hollow driven gear shaft  12  in a manner engageable with and disengageable from a hollow driven gear shaft  12  via swing pawl members  24, 25 . The transmission has first engaging means  25  for engaging the gear shaft  12  with a first driven gear n 1 , so as to effect synchronous rotation thereof; and second engaging means  25  for engaging the gear shaft  12  with a second gear n 2 , one step lower in reduction gear ratio than the first gear n 1 , so as to effect synchronous rotation thereof. When making an upshift by changing over the engaging means  25  from the engagement with the first gear n 1  to the engagement with the second gear n 2 , the gear shift is made by engaging the second gear n 2  with the gear shaft  12  through the second engaging means  25  while the first gear n 1  is being in engagement with the gear shaft  12  through the first engaging means  25.

TECHNICAL FIELD

The present invention relates to a multi-stage transmission in which aplurality of drive gears and a plurality of driven gears are supportedrespectively on parallel gear shafts in a state of being constantlymeshed to constitute meshing gear combinations for different speeds.

BACKGROUND ART

The constantly meshed type multi-stage transmission has a configurationin which one of the drive gears and the driven gears are fixed on a gearshaft, while the other of the drive gears and the driven gears arerotatably supported on another gear shaft, and in which one of therotatably supported gears is selected to be engaged with the other gearshaft by an engaging means, to make a gear shift.

There are patent publications disclosing a multi-stage transmission inwhich driven gears are rotatably supported on a driven gear shaft andengaging means are provided to make engagement and disengagement betweeneach of the driven gears and the driven gear shaft (refer to PatentDocument 1, for example).

[Patent Document 1]

Japanese Patent Publication No. 45-35687

The engaging means used in the constantly meshed type multi-stagetransmission disclosed in Patent Document 1 has such a structure that atubular body is provided which is formed with axially extending grooveseach having a widely cut hole extending in the circumferential directionat a middle portion of each groove, the tubular body is externallyfitted over a driven gear shaft so as to be movable in the axialdirection, and a plurality of driven gears are rotatably supported onthe tubular body.

The driven gear shaft has in the outer surface thereof flat cutouts ataxial positions corresponding to the driven gears. Rollers are putrespectively on each of the flat cutouts of the driven gear shaft, andare loosely fitted in the grooves and the widely cut holes of thetubular body, the rollers in each of the grooves being arranged in arow.

Regarding the driven gears rotated while being constantly meshed withthe drive gears, the rollers in the grooves do not make contact with theinner peripheral surfaces of the driven gears, so that rotation is nottransmitted from these driven gears, and only the rollers in the widelycut hole makes contact with and are engaged with the inner peripheralsurface of the relevant driven gear, so that rotation is transmittedfrom the relevant driven gear.

In other words, each of the driven gears at a position where the widelycut hole is located transmits rotation to the driven gear shaft, so thatgear shift can be achieved by moving the tubular body in the axialdirection.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In the engaging means used in the above-mentioned multi-stagetransmission, the widely cut hole for engaging the rollers with thedriven gear is formed with inclined surfaces at its inside edges so thatthe roller can be easily moved in the circumferential direction alongthe inclined surfaces. However, since the roller is firmly pressed orengaged between the flat cutout and the inner peripheral surface of thedriven gear, a considerable force is needed to release the pressedengagement. Therefore, it is not easy to smoothly move the tubular body,and, in some cases, a clutch for gear shifting may be needed.

In addition, time is needed for the changeover process in which movementof the tubular body, disengagement of a set of rollers from one drivengear through the function of the widely cut hole, movement of the widelycut hole to an adjacent set of rollers, and movement of these rollers inthe circumferential direction to be engaged with an adjacent drivengear, are effected sequentially for gear shift.

Besides, at the time of gear shift, a loss of the driving force isgenerated when the rollers are disengaged from the driven gear, and alarge shift shock is generated when the rollers are engaged with theadjacent driven gear, so that a smooth gear shift is difficult torealize.

The present invention has been made in consideration of theabove-mentioned problems. Accordingly, it is an object of the presentinvention to provide a multi-stage transmission which can be operatedwith small forces and in which there is no need for a clutch for gearshifting, there are no loss of changeover time for gear shift and noloss of the driving force, and the shift shock is small.

Means for Solving the Problems

In order to attain the above object, the present invention provides amulti-stage transmission wherein: a drive gear shaft and a driven gearshaft are disposed in parallel; a plurality of drive gears and aplurality of driven gears are supported on the drive gear shaft and thedriven gear shaft, respectively, in a manner that the drive gears andthe driven gears are constantly meshed with each other to constitutemeshing gear combinations for different speeds; one of the drive gearsand the driven gears are fixed to the gear shaft supporting the same,while the gear shaft supporting the other of the drive gears and thedriven gears is a hollow gear shaft; and the other gears are supportedon the hollow gear shaft individually in an engageable and disengageablemanner through engaging means, which are operated by gear shift drivingmeans for gear shift:

the engaging means includes: a first engaging device for causing a firstgear of the other gears and the hollow gear shaft to abut with eachother in a rotational direction of the gears to thereby cause the firstgear and the hollow gear shaft to be rotated synchronously; and a secondengaging device for causing a second gear, one step lower in reductiongear ratio than the first gear, of the other gears and the hollow gearshaft to abut with each other in the rotational direction of the gearsto thereby cause the second gear and the hollow gear shaft to be rotatedsynchronously; wherein for making an upshift through changing over theengaging means from the engagement with the first gear to the engagementwith the second gear by the gear shift driving means, the gear shift ismade by engaging the second gear with the hollow gear shaft through thesecond engaging device in a state in which the first gear is inengagement with the hollow gear shaft through the first engaging device.

According to a preferred form of the invention, the engaging meansfurther includes: a third engaging device for causing the first gear andthe hollow gear shaft to abut with each other in a counter-rotationaldirection of the gears to thereby cause the first gear and the hollowgear shaft to be rotated synchronously; and a fourth engaging device forcausing the second gear and the hollow gear shaft to abut with eachother in the counter-rotational direction of the gears to thereby causethe second gear and the hollow gear shaft to be rotated synchronously;wherein for making a downshift by changing over the engaging means fromthe engagement with the second gear to the engagement with the firstgear by the gear shift driving means under a driving force applied froma driven side, the gear shift is made by engaging the first gear withthe hollow gear shaft through the third engaging device in a state inwhich the second gear is in engagement with the hollow gear shaftthrough the fourth engaging device.

According to a still further preferred form of the invention, when thefirst gear is in engagement with the hollow gear shaft by the firstengaging device, the third engaging device is held in a state engageablewith the first gear, and for making an upshift from the first gear tothe second gear, the third engaging device is released from the stateengageable with the first gear before the second gear is engaged by thesecond engaging device; and when the second gear is in engagement withthe hollow gear shaft by the fourth engaging device, the second engagingdevice is held in a state engageable with the second gear, and at thetime of making a downshift from the second gear to the first gear, thesecond engaging device is released from the state engageable with thesecond gear before the first gear is engaged by the third engagingdevice.

In a preferred mode of the invention, each of the engaging devicescomprises: engaging projections provided at an inner peripheral surfaceof each of the gears at circumferentially spaced locations; a pluralityof cam rods which are put in sliding contact with an inner peripheralsurface of an internal hole of the hollow gear shaft so as to beslidable in an axial direction and which are provided in sliding contactsurfaces thereof with a plurality of cam grooves at axially spacedlocations; pin members which are fitted in through-holes radiallypenetrating portions of the hollow gear shaft and which are advanced andretracted while alternately making contact with the sliding contactsurfaces of the cam rods and the cam grooves; and swing pawl memberswhich are swingably supported in the hollow gear shaft and which areswung by advance/retreat of the pin members so as to be engaged with anddisengaged from the engaging projections; the cam rods being movable bythe gear shift driving means to thereby make a gear shift.

Preferably, the transmission includes drive source rotation deceleratingmeans for decelerating a drive source; and at the time of making adownshift under a driving force applied from a drive source, a downshiftoperation is conducted after deceleration by the drive source rotationdecelerating means.

As the drive source rotation decelerating means may be used one ofignition timing control means, fuel injection amount control means, andmotive power transmission control means by disengagement of a clutch.

Preferably, the multi-stage transmission further comprises means forpreventing an upshift in which the gear shift driving means changes overthe engaging means from the engagement with the first gear to theengagement with the second gear, when a driving force is being appliedfrom the driven side.

EFFECTS OF THE INVENTION

According to the constantly-meshed type multi-stage transmission of theinvention, at the time of making an upshift by changing over theengaging means from the engagement with the first gear to the engagementwith the second gear by the gear shift driving means, the gear shift ismade by engaging the second gear with the hollow gear shaft by thesecond engaging device while the first gear is in engagement with thehollow gear shaft through the first engaging device. In this case, owingto the difference in rotational speed between the gears, the engagementof the second gear with the hollow gear shaft by the second engagingdevice permits smooth release of the engagement of the first gear withthe hollow gear shaft through the first engaging device. Accordingly, noforce is needed to release the engagement, smooth operations areensured, and there is no need for a clutch for gear shifting. Inaddition, there are utterly no loss of changeover time for an upshiftand no loss of the driving force, the shift shock is small, and a smoothupshift can be achieved.

At the time of making a downshift by changing over the engaging meansfrom the engagement with the second gear to the engagement with thefirst gear by the gear shift driving means in response to a drivingforce applied from the driven side, the gear shift is made by engagingthe first gear with the hollow gear shaft through the third engagingdevice while the second gear is in engagement with the hollow gear shaftthrough the fourth engaging device. In this case, owing to thedifference in the rotational speed between the gears, the engagement ofthe first gear with the hollow gear shaft by the third engaging devicepermits smooth release of the engagement of the second gear with thehollow gear shaft through the fourth engaging device. Accordingly, noforce is needed to release the engagement, smooth operations areensured, and there is no need for a clutch for gear shifting. Besides,there are utterly no loss of changeover time at the time of a downshiftand no loss of the driving force, the shift shock is small, and a smoothupshift can be achieved.

When the first gear is in engagement with the hollow gear shaft throughthe first engaging device, the third engaging device is held in a stateengageable with the first gear. Therefore, even when a change occurs inthe direction of the driving force, the third engaging device havingbeen in the engageable state is speedily engaged with the first gear, sothat the engagement with the gear shaft is smoothly taken over andmaintained. Further, at the time of making an upshift from the firstgear to the second gear, the third engaging device is released from thestate engageable with the first gear before the second gear is engagedby the second engaging device. Therefore, smooth and reliable engagementand disengagement can be performed, and the upshift can be made, withoutbeing obstructed by the third engaging device.

Further, when the second gear is in engagement with the gear shaftthrough the fourth engaging device, the second engaging device is heldin a state engageable with the second gear. Therefore, even when achange occurs in the direction of the driving force, the second engagingdevice having been in the engageable state is speedily engaged with thesecond gear, so that the engagement with the gear shaft is smoothlytaken over and maintained. Furthermore, at the time of making adownshift from the second gear to the first gear, the second engagingdevice is released from the engageable state with the second gear beforethe first gear is engaged by the third engaging device. Accordingly,smooth and reliable engagement and disengagement can be performed, andthe downshift can be made without being obstructed by the secondengaging device.

By moving in the axial direction the cam rods which are put in slidingcontact with the inner peripheral surface of the internal hole of thehollow gear shaft and which are provided in their sliding contactsurfaces with the plurality of cam grooves at axially spaced locations,the pin members fitted in the portions of the hollow gear shaft areadvanced or retreated to swing the swing pawl members, whereby the swingpawl members are engaged with or disengaged from the engagingprojections of the gears. Therefore, gear shift can be made by changingover the engagement through advancing or retreating the related pinmembers with small amounts of movement of the cam rods. Accordingly, astructure is provided in which adjacent ones of the gears rotatablysupported on the hollow gear shaft are set close to each other, so thatthe size of the transmission in the axial direction can be reduced.

At the time of making a downshift under a driving force applied from adrive source, a downshift operation is conducted after deceleration bythe drive source rotation decelerating means. By doing so, theengagement with the second gear through the second engaging device canbe released by deceleration at the time of the gear shift. Accordingly,the engagement can be easily handed over to the engagement with thefirst gear through the first engaging device, whereby downshift can bemade even during acceleration.

As the drive source rotation decelerating means can be used ignitiontiming control means, fuel injection amount control means, or motivepower transmission control means by disengagement of a clutch. Thisensures that downshift can be easily performed even during acceleration.

When a driving force is being applied from the driven side, it isprohibited that the gear shift driving means changes over the engagementof the engaging means from the engagement with the first gear to theengagement with the second gear so as to make an upshift. By doing so,it is possible to eliminate the possibility that a shift shock isgenerated, upon engagement of the second gear by the engaging means, dueto acceleration occurring in response to an upshift operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a multi-stage transmission according toone embodiment of the present invention;

FIG. 2 illustrates a Geneva stop mechanism in gear shift driving means;

FIG. 3 illustrates the Geneva stop mechanism in another state;

FIG. 4 is a development of an outer peripheral surface of a shift drum;

FIG. 5 is a sectional view showing the structure of a counter gear shaftand surrounding elements thereof;

FIG. 6 is a sectional view taken along line VI-VI of FIG. 5;

FIG. 7 is a sectional view taken along line VII-VII of FIG. 5;

FIG. 8 is a sectional view taken along line VIII-VIII of FIG. 5;

FIG. 9 is a perspective view of a control rod;

FIG. 10 is an exploded perspective view of cam rods and lost motionmechanisms;

FIG. 11 is a perspective view showing a state in which the cam rods andthe lost motion mechanisms are mounted on the control rod;

FIG. 12 is an exploded perspective view of the counter gear shaft andpin members;

FIG. 13 shows perspective views of swing pawl members and pivot pins;

FIG. 14 is a perspective view showing a state in which the swing pawlmembers, the pivot pins, the cam rods, the control rods and so on aremounted on the counter gear shaft;

FIG. 15 is a perspective view showing a state in which one bearingcollar member has been externally fitted over the counter gear shaft inthe state shown in FIG. 14;

FIG. 16 illustrates a 1st gear speed condition at the time of startingan upshift;

FIG. 17 illustrates a step in the course of the upshift operation;

FIG. 18 illustrates a next step;

FIG. 19 illustrates a subsequent step;

FIG. 20 illustrates a further subsequent step;

FIG. 21 illustrates a 2nd gear speed condition upon completion of theupshift operation;

FIG. 22 illustrates the 2nd gear speed condition at the time of startinga downshift;

FIG. 23 illustrates a step in the course of the downshift operation; and

FIG. 24 illustrates the 1st gear speed condition upon completion of thedownshift.

DESCRIPTION OF REFERENCE SIGNS

m . . . Drive transmission gear; m1 to m5 . . . 1st to 5th drivetransmission gears; n . . . Driven transmission gear; n1 to n5 . . . 1stto 5th driven transmission gears; 5 . . . Frictional clutch;

10 . . . Multi-stage transmission; 11 . . . Main gear shaft; 12 . . .Counter gear shaft; 20 . . . Engaging means; 21 . . . First cam rod; 22. . . Second cam rod; 23 . . . Pin member; 24, 25 . . . Swing pawlmember; 25 c . . . Engaging claw part; 24 p, 25 p . . . Pin abuttingpart; 26 . . . Pivot pin; 27 . . . Torsion coil spring; 31 . . .Engaging projection; 50 . . . Gear shift driving means; 51 . . . Controlrod.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, an embodiment of the present invention will be described below withreference to FIGS. 1 to 24.

A multi-stage transmission 10 according to the present embodiment isused in combination with an internal combustion engine mounted on amotorcycle.

FIG. 1 is a sectional view of the multi-stage transmission 10. As shownin FIG. 1, the multi-stage transmission 10 is installed in an enginecase 1 which is common to the internal combustion engine.

The engine case 1 is of a left-right split type and is made by couplinga left engine case 1L and a right engine case 1R. The engine case 1defines a transmission chamber 2, in which a main gear shaft 11 and acounter gear shaft 12 are rotatably supported in parallel to each otherand in left-right orientation.

The main gear shaft 11 is rotatably supported in side walls of the leftengine case 1L and the right engine case 1R through bearings 3L and 3R,respectively, and a multiple-disk friction clutch 5 is provided at aright end part of the main gear shaft 11, which protrudes from thetransmission chamber 2 by penetrating the right bearing 3R.

On the left side of the friction clutch 5, a primary driven gear 4, towhich rotation of a crankshaft (not shown) is transmitted, is rotatablysupported on the main gear shaft 11.

The friction clutch 5 has a structure in which a clutch inner element 5i is spline-fitted over a right end part of the main gear shaft 11 andfixed to the right end part by a nut 7, and in which a large-diameter,bowl-shaped clutch outer element 5 o is connected to the primary drivengear 4 by connecting means 6, the clutch outer element 5 o accommodatingtherein the clutch inner element 5 i and a pressure plate 5 p combinedwith the clutch inner element 5 i.

Therefore, the rotation of the crankshaft of the internal combustionengine is transmitted from the primary driven gear 4 to the main gearshaft 11 through the friction clutch 5 in an engaged (connected) statethereof.

On the other hand, the counter gear shaft 12 is also rotatably supportedin the side walls of the left engine case 1L and the right engine case1R through bearings 7L and 7R, respectively, and an output sprocket 8 isfixed by spline-fit to a left end part of the counter gear shaft 12,protruding from the transmission chamber 2 by penetrating the leftbearing 7L.

A drive chain 9 wrapped around the output sprocket 8 is wrapped around asprocket for driving a rear wheel (not shown) of the motorcycle, wherebythe rotational power of the counter gear shaft 12 is transmitted to therear wheel, and the motorcycle is made to run.

Between the left and right bearings 3L and 3R, a group of drivetransmission gears (m) are provided on the main drive shaft 11 so thatthey can each be rotated as one body with the main gear shaft 11.

A 1st drive transmission gear m1 is formed as one body with the maingear shaft 11 along the right bearing 3R, and 3rd, 5th, 4th, and 2nddrive transmission gears m3, m5, m4, and m2 in this order from the rightto the left side are fixed by spline-fit to the main gear shaft 11through splines extending between the 1st drive transmission gear m1 andthe left bearing 3L.

On the other hand, a group of driven transmission gears (n) aresupported on the counter gear shaft 12 between the left and rightbearings 7L and 7R through annular bearing collar members 13therebetween so that they can be rotated relative to the main gear shaft11.

On the counter gear shaft 12, the bearing collar member 13 at the rightend is externally mounted through a collar ring 14R fitted on thecounter gear shaft 12 at the left side of the right bearing 7R, whilethe bearing collar member 13 at the left end is externally mounted onthe counter gear shaft 12 through a collar ring 14L fitted on thecounter gear shaft 12 at the right of the left bearing 7L. In addition,four bearing collar members 13 are externally mounted on the countergear shaft 12 at regular intervals between the right-end bearing collarmember 13 and the left-end bearing collar member 13, and 1st, 3rd, 5th,4th, and 2nd driven transmission gears n1, n3, n5, n4, and n2 in thisorder from the right to the left side are rotatably supported on thecounter gear shaft 12 in such a manner that these driven transmissiongears bridge adjacent ones of the total of six bearing collar members13.

The 1st, 3rd, 5th, 4th, and 2nd drive transmission gears m1, m3, m5, m4,and m2 each rotated as one body with the main gear shaft 11 are normallymeshed respectively with the corresponding 1st, 3rd, 5th, 4th, and 2nddriven transmission gears n1, n3, n5, n4, and n2 which are rotatablysupported on the counter gear shaft 12.

The meshing of the 1st drive transmission gear m1 with the 1st driventransmission gear n1 constitutes the 1st gear speed corresponding to ahighest reduction gear ratio, while the meshing of the 5th drivetransmission gear m5 with the 5th driven transmission gear n5constitutes the 5th gear speed corresponding to a lowest reduction gearratio, and the reduction gear ratio is sequentially decreasedtherebetween to constitute the 2nd, 3rd, and 4th gear speeds.

The counter gear shaft 12, which is tubular in shape, is providedtherein with engaging means 20 (FIG. 13) capable of engaging with eachof the driven transmission gears (n), as will be described later indetail. A total of four cam rods, that is, first and second cam rods 21,21 and 22, 22 as components of the engaging means 20 are axiallyslidably fitted in the inner peripheral surface of the counter gearshaft 12.

As shown in FIG. 10, the four first and second cam rods 21, 21 and 22,22 are substantially in such shapes that a hollow cylinder is evenlydivided into four sections arrayed in the circumferential direction; thecam rods are classified into two kinds, i.e., the first cam rods 21 andthe second cam rods 22, and each pair of the cam rods of the same kindare located at diametrically symmetrical positions.

The first and second cam rods 21 and 22 are cam members of which thesliding contact surfaces in sliding contact with the inner peripheralsurface of the counter gear shaft 12 constitute cam surfaces. The firstand second cam rods 21 and 22 are provided with circumferentiallyextending arcuate cam grooves 21 v and 22 v respectively at fiverequired locations along the axial direction of the counter gear shaft.

A control rod 51 is a component of gear shift driving means 50 fordriving the first and second cam rods 21 and 22 by gear shift. Thecontrol rod 51 is inserted in the counter gear shaft 12 concentricallytherewith and in sliding contact with the inner surfaces of the fourfirst and second cam rods 21, 21 and 22, 22. Movement of the control rod51 in the axial direction causes the first and second cam rod 21, 21 and22, 22 to also move through lost motion mechanisms 52 and 53,respectively.

A mechanism for moving the control rod 51 in the axial direction isprovided on the left side of the left engine case 1L.

A power transmission case 80 is provided on the left side of the leftengine case 1L so as to cover the output sprocket 8, and the left sideof the power transmission case 80 is covered with a case cover 81, so asto define a gear shift driving chamber 82 between the power transmissioncase 80 and the case cover 81.

The left end of the counter gear shaft 12 is fitted in an opening in thepower transmission case 80 through a seal member 83 so as to face thegear shift driving chamber 82, and the control rod 51 in the countergear shaft 12 projects leftwards beyond the left end of the counter gearshaft 12, into the gear shift driving chamber 82.

On the upper side of the power transmission case 80 is firmly attached agear shift driving motor 60, with its drive shaft 60 a protruding intothe gear shift driving chamber 82 on the left side.

A Geneva driving gear 62 is provided between the power transmission case80 and the case cover 81 and has its integral rotary shaft 62 arotatably supported by bearings 61 and 61. Teeth 62 g of the Genevadriving gear 62 are meshed with drive gear teeth 60 g formed on thedrive shaft 60 a of the gear shift driving motor 60.

The Geneva driving gear 62 is provided with an operating pin 62 pprojecting leftwards at a position offset by a predetermined distancefrom the center of rotation of the Geneva driving gear 62. The Genevadriving gear 62 is formed with a raised surface 62 s of an arcuate shapeof a predetermined radius on the opposite side, with respect to thecenter of rotation, from the operating pin 62 p (see FIGS. 1, 2 and 3).

Besides, in the gear shift driving chamber 82, between the Genevadriving gear 62 and the left end part of the control rod 51, a shiftdrum 65 is provided between the power transmission case 80 and the casecover 81, with a rotary shaft 65 a of the shift drum 65 rotatablysupported by bearings 64 and 64.

The shift drum 65 is provided with a guide groove 65 v in the outerperipheral surface of the drum.

FIG. 4 shows a development of the outer peripheral surface of the shiftdrum 65. The guide groove 65 v is so formed that a neutral (N) positionis provided at a position toward the left side, a 1st gear speedposition apart by 60 degrees along the circumferential direction fromthe neutral (N) position is provided at a position displaced to theright side in the axial direction relative to the neutral (N) position,and 2nd, 3rd, 4th, and 5th gear speed positions apart by 60 degrees eachfrom the 1st gear speed position are successively provided at positionssequentially displaced to the right side in the axial direction from the1st gear speed position.

A Geneva driven gear 63 is fitted on the rotary shaft 65 a of the shiftdrum 65, correspondingly to the Geneva driving gear 62.

As shown in FIGS. 2 and 3, the Geneva driven gear 63 is provided withsix radially orientated grooves 63 p at regular angular intervals of 60degrees in the circumferential direction, and an arcuate recessedsurface 63 s is formed between each pair of adjacent radial grooves 63 pand 63 p.

The Geneva driving gear 62 and the Geneva driven gear 63 constitute a⅙-revolution Geneva stop mechanism.

Specifically, as shown in FIG. 2, from the moment when the operating pin62 p turned by the rotation of the Geneva driving gear 62 enters oneradial groove 63 p in the Geneva driven gear 63 until it comes out ofthe radial groove 63 p, it rotates the Geneva driven gear 63 by ⅙revolution. At the time when the operating pin 62 p comes out of theradial groove 63 p, the arcuate raised surface 62 s of the Genevadriving gear 62 is engaged with the arcuate recessed surface 63 s of theGeneva driven gear 63, whereby the Geneva driven gear 63 is locked andfixed, as shown in FIG. 3.

Therefore, with one revolution of the Geneva driving gear 62, the Genevadriven gear 63 is securely rotated by ⅙ revolution as one body with theshift drum 65.

Referring to FIG. 1, the inner races of two ball bearings 71 and 71 arefirmly attached to the left end part of the control rod 51 by a nut 72,and a control rod operating element 70 is fitted over the outer races ofthe ball bearings 71 and 71.

An engaging pin 75 projecting from the control rod operating element 70is in sliding engagement with an elongate groove 80 v formed in thepower transmission case 80 to extend in the left-right direction.

In addition, separately from this, a guide pin 76 is provided on thecontrol rod operating element 70 to project toward the shift drum 65,and is in sliding engagement with the guide groove 65 v in the shiftdrum 65.

Therefore, the control rod operating element 70 rotatably holds the leftend part of the control rod 51 while being restricted in its ownrotation due to the engagement of the engaging pin 75 with the elongatedgroove 80 v.

When the shift drum 65 is intermittently rotated as above-mentioned, thecontrol rod operating element 70 is moved along the left-right axialdirection through the guide pin 76 engaged with the guide groove 65 v inthe shift drum 65, and therefore, the movement of the control rodoperating element 70 moves the control rod 51 by a required amount inthe axial direction through the two ball bearings 71 and 71.

The movement of the control rod 51 in the axial direction moves the camrods 21 and 22 through the lost motion mechanism 52 and 53,respectively, and the movement of the cam rods 21 and 22 causes theengaging means 20 incorporated in the counter gear shaft 12 toselectively engage one of the driven transmission gears (n) with thecounter gear shaft 12, to achieve a gear shift for establishing a newtransmission condition).

Now, the structure of the engaging means 20 will be described below withreference to FIGS. 5 to 15.

First, the control rod 51 of the gear shift driving means 50 will bedescribed referring to FIG. 9.

The control rod 51 is provided with an elongated central cylindricalpart 51 a having an outer diameter for sliding contact with the insidesurfaces of the four cam rods 21, 21 and 22, 22. The control rod 51 isfurther provided on the left side in sequential order with areduced-diameter part 51 bb, a spline part 51 s having spline teeth 51ss, and a left-end cylindrical part 51 b, and on the right side insequential order with a reduced-diameter part 51 cc and a right-endcylindrical part 51 c in which an engaging groove 51 v oriented in theaxial direction is cut.

The maximum diameter of the spline teeth 51 ss of the spline part 51 sis approximately equal to the inner diameter of the counter gear shaft12.

Referring to FIG. 10, the four first and second cam rods 21, 21 and 22,22 in sliding contact with the outer surface of the central cylindricalpart 51 a of the control rod 51 are, as mentioned above, substantiallyin such shapes that a hollow cylinder is evenly divided into foursections arrayed in the circumferential direction. The outer peripheralsurfaces of the first and second cam rods 21, 21 and 22, 22 as thesliding contact surfaces in sliding contact with the inner peripheralsurface of the counter gear shaft 12 are provided with the arcuate camgrooves 21 v and 22 v extending circumferentially of the cam rods atfive spaced-apart locations along the axial direction.

The cam grooves 21 v and 22 v are each so formed that the opposite sidesurfaces extending from the groove bottom surface are appropriatelyinclined so that the cam grooves open wider toward the outer side.

Mutually adjoining opposite side edges of the outer peripheral surfacesof the first and second cam rods 21 and 22 are cut in a slanted mannerso that, when adjacent cam rods come into contact with each other, anaxial notch (v) with a V-shaped cross section is formed betweenadjoining cam rods (see FIG. 11).

The two first cam rods 21 and 21 of the same kind are located atdiametrically symmetrical positions and have right ends slightlyextended axially and provided with lock claws 21 c and 21 c, while thetwo second cam rods 22 and 22 have left ends slightly extended axiallyand provided with lock claws 22 c and 22 c.

The lost motion mechanism 52 on the right side has a spring 52 sretained, as shown in FIG. 5, between an inner peripheral recessed partof a hollow cylindrical spring holder 52 h. The spring 52 s is slidablyfitted around the right-side reduced-diameter part 51 cc of the controlrod 51, and is clamped between split cotters 52 c and 52 c on bothsides, which are fitted in both the inner peripheral recessed part andthe reduced-diameter part 51 cc.

The left end surface of the spring holder 52 h opposed to the two firstcam rods 21 and 21 projects axially to form lock claws 52 hc and 52 hcfor locking engagement with the lock claws 21 c and 21 c of the firstcam rods 21 and 21.

Similarly, as shown in FIG. 5, the lost motion mechanism 53 on the leftside has a spring 53 s retained in an inner peripheral recessed part ofa hollow cylindrical spring holder 53 h, and the spring 53 s is slidablyfitted around the left-side reduced-diameter part 51 bb of the controlrod 51. The spring 53 s is clamped between a split cotter 53 c and awasher 53 w on both sides, which are fitted in both the inner peripheralrecessed part and the reduced-diameter part 51 bb.

The right end surface of the spring holder 53 h opposed to the twosecond cam rods 22 and 22 projects axially to form lock claws 53 hc and53 hc for locking engagement with the lock claws 22 c and 22 c of thesecond cam rods 22 and 22.

As shown in FIGS. 10 and 11, the outer peripheral surfaces of the springholders 52 h and 53 h are provided with notch grooves 52 hv and 53 hv,respectively. The notch grooves 52 hv and 53 hv are V-shaped in crosssection and provided at four positions at regular angular intervalsalong the circumferential direction, correspondingly to the notches inthe outer peripheral surfaces of the first and second cam rods 21 and22.

As shown in FIG. 11, the lost motion mechanisms 53 and 52 are mounted tothe left and right reduced-diameter parts 51 bb and 51 cc of the controlrod 51, respectively, and the four first and second cam rods 21, 21 and22, 22 are put in sliding contact with the outer periphery of thecentral cylindrical part 51 a between the spring holders 52 h and 53 h.

In this case, the first and second cam rods 21 and 22 of the differentkinds are circumferentially alternately disposed so that each pair ofthe cam rods of the same kind are located at diametrically symmetricalpositions. Besides, the first cam rods 21 and 21 have their lock claws21 c and 21 c at their right ends in locking engagement with the lockclaws 52 hc and 52 hc of the spring holder 52 h on the right side, whilethe second cam rods 22 and 22 have their lock pawls 22 c and 22 c attheir left ends in locking engagement with the lock claws 53 hc and 53hc of the spring holder 53 h on the left side.

In the state where the four first and second cam rods 21, 21 and 22, 22are mounted on the periphery of the control rod 51 together with thelost motion mechanisms 52 and 53 in the above manner, the assembly isfitted into the internal hole of the hollow cylindrical counter gearshaft 12.

The inner peripheral surface of the counter gear shaft 12 is providedwith axially oriented slightly projected ribs at four positions atregular intervals along the circumferential direction, at portionscorresponding to the first and second cam rods 21, 21 and 22, 22 and thespring holders 52 h and 53 h. The first and second cam rods 21, 21 and22, 22 and the spring holders 52 h and 53 h have the V-shaped notches(v) and the notch grooves 52 hv and 53 hv, which are fitted over theprojected ribs of the inner peripheral surface of the counter gear shaft12, whereby they are positioned and restrained from relative rotation inthe circumferential direction while they are kept in sliding contact soas to be movable in the axial direction only.

Referring to FIGS. 5 and 6, the inner peripheral surface of the countergear shaft 12 is provided, at its portion corresponding to the splinepart 51 s of the control rod 51, with spline teeth 12 ss to be meshedwith the spline teeth 51 ss on the side of the control rod 51, wherebythe control rod 51 is put in sliding contact so as to be movable in theaxial direction while being restrained from rotation, relative to thecounter gear shaft 12.

While eight spline teeth 51 ss are provided on the control rod 51, onlyfour spline teeth 12 ss are provided on the counter gear shaft 12, andthe portions of the missing spline teeth form communicating holes whichserve as lubricating oil passages 51 o.

Furthermore, as shown in FIG. 5, an annular collar member 54press-fitted into a right-end opening of the counter gear shaft 12 holdsa right-end cylindrical part 51 c of the control rod 51, and an engagingpin 55 projecting from the collar member 54 toward the center is engagedin an axial engaging groove 51 v cut in the right-end cylindrical part51 c.

Therefore, the control rod 51 is so supported that it is movable in theaxial direction while restrained from rotation, due to the engagement atthe opposite ends by the above-mentioned anti-rotation mechanisms,relative to the counter gear shaft 12.

When the control rod 51 and the lost motion mechanisms 52 and 53 and thefour first and second cam rods 21, 21 and 22, 22 are put in the internalhole of the counter gear shaft 12, all these components are rotatedtogether. Besides, when the control rod 51 is moved in the axialdirection, the first cam rods 21 and 21 are moved in the axial directionrelatively to the counter gear shaft 12 through the spring 52 s of thelost motion mechanism 52, and the second cam rods 22 and 22 are moved inthe axial direction relatively to the counter gear shaft 12 through thespring 53 s of the lost motion mechanism 53.

As shown in the perspective view of FIG. 12, the hollow cylindricalcounter gear shaft 12 is provided with a left-side cylindrical part 12 band a right-side cylindrical part 12 c which are reduced in outerdiameter and are located respectively on both the left and right sidesof the central cylindrical part 12 a. The central cylindrical part 12 acontains the first and second cam rods 21, 21 and 22, 22 therein andsupports the driven transmission gears (n) thereon.

A bearing 7L is fitted over the left-side cylindrical part 12 b througha washer 56 (see FIG. 5), and the left-side cylindrical part 12 b ispartly provided with splines 12 bb with which the output sprocket 8 isspline-engaged.

In addition, a bearing 7R is fitted over the right-side cylindrical part12 c through a washer 57 (see FIG. 5).

As shown in FIG. 12, the central cylindrical part 12 a of the countergear shaft 12 is enlarged in outer diameter and has an increasedthickness. The thick outer peripheral part is provided with five annularcircumferential grooves 12 cv at regular intervals along the axialdirection, and with four axially oriented axial grooves 12 av at regularintervals along the circumferential direction.

A plurality of sections are demarcated by the five circumferentialgrooves 12 cv and the four axial grooves 12 av, in the outer peripheralpart of the central cylindrical part 12 a of the counter gear shaft 12.Substantially rectangular recessed parts 12 d are formed in some of theplurality of sections.

Each of the annular parts demarcated by the circumferential grooves 12cv is demarcated by the four axial grooves 12 av into four sections, ofwhich two diametrically symmetrical ones are provided with the recessedparts 12 d, and each of the recessed parts 12 d shares a common spacewith the circumferential groove 12 cv at the left side thereof.

As shown in FIG. 12, two recessed parts 12 d on the left side in theaxial direction and three recessed parts 12 d on the right side arearrayed in rows, with a circumferential stagger of 90 degreestherebetween.

The bottom part of each of the circumferential grooves 12 cv is providedwith radially extending pin holes 12 h at four circumferentiallyspaced-apart locations, and pin members 23 are inserted in the pin holes12 h so that they can radially advance and recede.

The radial pin hole 12 h opens in both the inner peripheral surface ofthe interior of the counter gear shaft 12 and the bottom surface of thecircumferential groove 12 cv, and the pin member 23 inserted in the pinhole 12 h can come into and out of the interior of the counter gearshaft 12 and can advance and recede while projecting into thecircumferential groove 12 cv.

The first and second cam rods 21 and 22 are in sliding contact with theinner peripheral surface of the counter gear shaft 12. Therefore, whenthe cam grooves 21 v and 22 v in the sliding contact surfaces of thefirst and second cam rods 21 and 22 are moved to such positions as toface the pin holes 12 h, the pin members 23 drop into the cam grooves 21v and 22 v, so that the amount of projection thereof into thecircumferential groove 12 cv is reduced. When the sliding surfaces ofthe first and second cam rods move to such positions as to face the pinholes 12 h, the pin members 23 move out of the cam grooves 21 v and 22v, so that the amount of projection thereof into the circumferentialgroove 12 cv is increased.

As indicated in FIGS. 17 and 18, two kinds of swing pawl members 24 and25 are put in the recessed parts 12 d and the circumferential grooves 12cv in the counter gear shaft 12, and pivot pins 26 for swingablysupporting the swing pawl members 24 and 25 are fitted in the axialgrooves 12 av.

FIG. 13 shows, in exploded perspective, the four swing pawl members 24,24 and 25, 25, which are put in the rightmost circumferential groove 12cv and the recessed part 12 d on the right side thereof, and the pivotpins 26 therefor, and which are put in the leftmost circumferentialgroove 12 cv and the recessed part 12 d on the right side thereof andthe pivot pins 26 therefor.

The swing pawl member 24 and 25 are substantially arcuate in shape asviewed in the axial direction, and are provided in their central partswith bearing recessed parts 24 d and 25 d, respectively, in the form ofa through-hole whose outer circumferential part is partly lost, throughwhich the pivot pins 26 are passed, respectively. Pin abutting parts 24p and 25 p are extended on one side of the swinging center of thebearing recessed parts 24 d and 25 d, respectively and engaging clawparts 24 c and 25 c are extended on the other side.

The pin abutting parts 24 p and 25 p have a reduced axial width forswingable fit in the circumferential groove 12 cv, and are provided intheir inner peripheral surfaces with pin receiving recesses 24 pp and 25pp for receiving the pin members 24 p and 25 p, respectively. Theengaging claw parts 24 c and 25 c on the other side have an enlargedaxial width for swingable fit in the recessed part 12 d, and are formedin a convergent shape as viewed in the axial direction.

The enlarged-width engaging claw parts 24 c and 25 c are heavier thanthe reduced-width pin members 24 p and 25 p, so that they act as apendulum in turning, so as to swing the swing pawl members 24 and 25 bycentrifugal force.

The two kinds of the swing pawl members 24 and 25 are diametricallysymmetrical.

The pivot pins 26 are fitted in the bearing recessed parts 24 d and 25 dso as to swingably support the swing claw members 24 and 25.

The swing pawl members 24 and 25 are put in the recessed parts 12 d andthe circumferential grooves 12 cv in the counter gear shaft 12, whilethe pivot pins 26 are fitted in the bearing recessed parts 24 d and 25 dof the swing pawl members 24 and 25 and also fitted and retained in theaxial grooves 12 av.

Referring to FIGS. 7 and 8, in each recessed part 12 d of the countergear shaft 12, the swing pawl members 24 and 25 adjacent to each otherin the circumferential direction are put such that the tips of theengaging claw parts 24 c and 25 c thereof are opposite to each otherwith a predetermined gap therebetween.

As shown in FIG. 13, torsion coil springs 27 are put around the pivotpins 26, respectively. The torsion coil springs 27 are so mounted thatone ends of the torsion coil springs 27 are engaged from inside with theengaging claw part 24 c and 25 c of the swing pawl members 24 and 25,respectively, and the other ends are in contact with the bottom surfacesof the recessed parts 12 d, whereby the swing pawl member 24 and 25 areurged so as to swing the engaging claw part 24 c and 25 c toward theoutside.

The pin members 23 act from radially inside to push radially outward thepin abutting parts 24 p and 25 p on one side of the swing pawl members24 and 25, and, therefore, the pin members 23 operate to swing the swingpawl member 24 and 25 against the urging forces exerted by the torsioncoil springs 27.

Now, the procedure for mounting the above-described engaging means 20 tothe counter gear shaft 12 will be described below.

First, as above-mentioned, the lost motion mechanisms 52 and 53 and thefour first and second cam rods 21, 21 and 22, 22 are assembled on theperipheral surface of the control rod 51 to obtain the assembledcondition as shown in FIG. 11, and the assembly is inserted into theinternal hole of the counter gear shaft 12.

In this case, the first and second cam rods 21, 21 and 22, 22 are sopositioned as to correspond in circumferential position respectively tothe axial grooves 12 av in the counter gear shaft 12, and the pin holes12 h are situated to face the sliding contact surfaces of the first andsecond cam rods 21, 21 and 22, 22 (see FIGS. 7 and 8).

A setting is made such that the moving position in the left-rightdirection of each of the four first and second cam rods 21, 21 and 22,22 in relation to the counter gear shaft 12 is in a neutral position.

When the pin members 23 are inserted in the pin holes 12 h formed in thecircumferential grooves 12 cv in the counter gear shaft 12 under thiscondition, all the pin members 23 abut on the sliding contact surfacesof the first and second cam rods 21, 21 and 22, 22, so that the amountof projection of the pin members 13 from the circumferential grooves 12cv is set to be large.

Then, in the condition where the pivot pins 26 with the torsion coilsprings 27 put thereon are fitted in the bearing recessed parts 24 d and25 d of the swing pawl members 24 and 25, the swing pawl members 24 and25 and so on are put in the recesses such as the recessed parts 12 d,the circumferential grooves 12 cv, the axial grooves 12 av, etc. formedin the outer periphery of the counter gear shaft 12.

The swing pawl member 24 and 25 are in such a condition that the pinreceiving parts 24 p and 25 p are pushed up from inside by the largelyprojecting pin members 23 against the urging forces exerted by thetorsion coil springs 27, so as to pull the engaging claw part 24 c and25 c inwards. Therefore, as shown in FIG. 14, no components areprojecting outwards from the outer peripheral surface of the centralcylindrical part 12 a of the counter gear shaft 12.

When the pin members 23 drop into the cam groove 21 v and 22 v in therelevant ones of the first and second cam rods 21 and 22, the swing pawlmembers 24 and 25 are swung about the pivot pins 26 by the urging forcesof the torsion coil springs 27 and the centrifugal forces of theengaging claw parts 24 c and 25 c heavier than the pin receiving parts24 p and 25 p, so that the engaging claw parts 24 c and 25 c areprojected outwards beyond the outer peripheral surface of the centralcylindrical part 12 a of the counter gear shaft 12.

In the condition where the engaging means 20 are assembled in thecounter gear shaft 12 shown in FIG. 14 and no components protrudeoutwards beyond the outer peripheral surface of the central cylindricalpart 12 a, the bearing collar members 13 are sequentially fitted overthe assembly.

The bearing collar members 13 are externally fitted on the centralcylindrical part 12 a at axial positions other than the positions of therecessed parts 12 d, and are disposed to bridge axially adjacent ones ofthe pivot pins 26 fitted in the axial grooves 12 av in succession and ina row, thereby preventing the pivot pins 26 and the swing pawl members24 and 25 from moving out of position.

The pivot pins 26 fitted in the axial grooves 12 av in the centralcylindrical part 12 a of the counter gear shaft 12 have such a size asto be flush with the outer peripheral surface of the central cylindricalpart 12 a, so that the pivot pins 26 are fixed without any clearanceonce the bearing collar members 13 are fitted externally.

The six bearing collar members 13 are externally fitted over the countergear shaft 12 at equal intervals, and the driven transmission gears (n)are rotatably supported thereon in a manner to extend over adjacent oneof the bearing collar members 13.

Each driven transmission gear (n) is formed with annular notches at itsleft and right inner peripheral edge parts, so that an annularprotrusion 30 of small height is formed between the left and rightnotches, and adjacent ones of the bearing collar members 13 are slidablyengaged with the annular notches in a manner to grip the protrusion 30therebetween (see FIGS. 5, 7 and 8).

The protrusion 30 on the inner peripheral surface of each driventransmission gear (n) is formed with engaging projections 31 at sixlocations at regular intervals along the circumferential direction.

Each engaging projection 31 is arcuate in shape in side view (as viewedin the axial direction in FIGS. 7 and 8), and opposite side edgesthereof in the circumferential direction constitute engaging surfacesfor engagement with the engaging claw parts 24 c and 25 c of the swingpawl members 24 and 25.

The swing pawl member 24 and the swing pawl member 25 have theirengaging claw parts 24 c and 25 c in mutually confronting positions, andthe swing pawl member 24 is adapted to abut on and be engaged with theengaging projection 31 in the rotational direction of the driventransmission gears (n) (and the counter gear shaft 12), whereas theswing pawl member 25 is adapted to abut on and be engaged with theengaging projection 31 in the counter-rotational direction of the driventransmission gears (n).

Incidentally, the swing pawl member 24 does not engage with the engagingprojection 31 in the counter-rotational direction of the driventransmission gears (n) even if its engaging claw part 24 c is projectingoutwards. Similarly, the swing pawl member 25 does not engage with theengaging projection 31 in the rotational direction of the driventransmission gears (n) even if its engaging claw part 25 c is projectingoutwards.

In practice, the process of assembling the engaging means 20 and thelike into the counter gear shaft 12 and mounting the five driventransmission gears (n) is carried out as follows.

First, the control rod 51, the cam rods 21 and 22 and the lost motionmechanisms 52 and 53 assembled together in the state shown in FIG. 11are inserted into the counter gear shaft 12 as above-mentioned, whilethe pin members 23 are preliminarily inserted in the pin holes 12 h.This assembly is then erected with the left portion upward.

Then, first, the right-end bearing collar member 13 is fitted over thelower end (right end) of the central cylindrical part 12 a as indicatedby solid lines in FIG. 15, and the engaging means 20 (the swing pawlmembers 24, 25, the pivot pins 26, and the torsion coil springs 27) forthe 1st driven transmission gear n1 is inserted into position. Next, the1st driven transmission gear n1 is fitted over from above to be mountedinto position so that the protrusion 30 of the 1st driven transmissiongear n1 abuts on the bearing collar member 13 and the notch is engagedwith the bearing collar member 13. Thereafter, the second bearing collarmember 13 is fitted over from above to be engaged with the notch of the1st driven transmission gear n1, whereby the second bearing collarmember 13 is externally mounted in position on the counter gear shaft 12and the 1st driven transmission gear n1 is supported in position in theaxial direction.

Next, the engaging means 20 for the 3rd driven transmission gear n3 isinserted in position, the 3rd driven transmission gear n3 is mounted inposition, and, then such procedure is repeated to sequentially mount theremaining 5th, 4th, and 2nd driven transmission gears n5, n4, n2 inrespective positions, followed finally by externally mounting the sixthbearing collar member 13.

In the condition where the five driven transmission gears (n) are thusmounted on the counter gear shaft 12, the counter gear shaft 12 isrotatably supported in the left and right bearings 7L and 7R attached tothe side walls of the left engine case 1L and the right engine case 1Rin the manner of being clamped therebetween through the collar members14L and 14R, as shown in FIG. 1. As a result, the five driventransmission gears (n) and the six bearing collar members 13 are clampedfrom the left and right sides in an alternately arranged state and areaxially positioned correctly.

The bearing collar members 13 can bear the axial forces concerning thedriven transmission gears (n), and can therefore ensure positioning ofthe driven transmission gears (n) in the axial direction and bear thrustforces therefrom.

In this manner, the 1st, 3rd, 5th, 4th, and 2nd driven transmissiongears n1, n3, n5, n4, n2 are rotatably supported on the counter gearshaft 12 through the bearing collar members 13.

Since the first and second cam rods 21 and 22 are each in the neutralposition, all the driven transmission gears (n) are in a disengagedcondition in which, due to the moving positions of the first and secondcam rods 21 and 22 of the corresponding engaging means 20, the pinmembers 23 are projected to push up from inside the pin abutting parts24 p and 25 p of the swing pawl members 24 and 25 and to allow theengaging claw parts 24 c and 25 c to swing inwards; and thus, all thedriven transmission gears (n) are freely rotatable in relation to thecounter gear shaft 12.

On the other hand, in an engaged condition in which, due to the movingpositions of the cam rods 21 and 22 of the engaging means 20, other thanthe neutral positions, the pin members 23 enter the cam grooves 21 v and22 v and the swing pawl members 24 and 25 are swung so that the engagingclaw parts 24 c and 25 c are projected outwards, and the engagingprojections 31 of the relevant driven transmission gear (n) abut againstthe engaging claw parts 24 c and 25 c. As a result, the rotation of therelevant driven transmission gear (n) is transmitted to the counter gearshaft 12, or the rotation of the counter gear shaft 12 is transmitted tothe relevant driven transmission gear (n).

When the gear shift driving motor 60 of the above-mentioned gear shiftdriving means 50 is driven and the Geneva driving gear 62 is rotated byone revolution, the shift drum 65 is rotated by 60 degrees by the⅙-revolution Geneva stop mechanism, and the guide pin 76 engaged withthe guide groove 65 v in the shift drum 65 is guided to move the controlrod operating element 70, which is integral with the guide pin 76, by apredetermined amount in the axial direction.

The control rod operating element 70 moves the control rod 51 togetherby the predetermined amount in the axial direction through the two ballbearings 71. The movement of the control rod 51 in the axial directionmoves, in conjunction, the first and second cam rods 21 and 22 in theaxial direction through the springs 52 s and 53 s of the lost motionmechanisms 52 and 53.

With the first and second cam rods 21 and 22 thus moved in the axialdirection, the pin members 23 in sliding contact with the first andsecond cam rods 21 and 22 in the engaging means 20 move into or out ofthe cam grooves 21 v and 22 v to swing the swing pawl members 24 and 25so that the latter are disengaged from one driven transmission gear (n)and engaged with another driven transmission gear (n), whereby thedriven transmission gear (n) engaged with the counter gear shaft 12 ischanged over, i.e., a gear shift is achieved.

While the gear shift driving motor 60 to make gear shifts is used as thegear shift driving means, a mechanism may be adopted in which the driveroperates a gear shift lever or the like to advance or retreat a cable,whereby the shift drum is rotated through the function of a Geneva stopmechanism or the like, thereby achieving a gear shift.

Now, the process of an upshift from the 1st gear speed condition to the2nd gear speed condition, which is one step lower in reduction gearratio than the 1st gear speed condition, at the time of acceleration bydriving the internal combustion engine will be described below,referring to FIGS. 16 to 21.

FIGS. 16 to 21 illustrate changes with time, and in each of the figures,(a) is a sectional view showing the structure of the counter gear shaft12 and the surrounding elements therefor, (b) is a sectional view(sectional view of 2nd driven transmission gear n2) taken along line b-bof (a), and (c) is a sectional view (sectional view of the 1st driventransmission gear n1) taken along line c-c of (a).

The motive power of the internal combustion engine is transmittedthrough the frictional clutch 5 to the main gear shaft 11, whereby the1st, 3rd, 5th, 4th, and 2nd drive transmission gears m1, m3, m5, m4, m2are rotated as one body, and the 1st, 3rd, 5th, 4th, and 2nd driventransmission gears n1, n3, n5, n4, n2 constantly meshed respectivelywith them are thereby rotated at respective rotational speeds.

FIG. 16 illustrates the 1st gear speed condition. In FIG. 16( b), the2nd driven transmission gear n2 is being rotated in the direction of thearrow, and, in FIG. 16( c), the 1st driven transmission gear n1 isrotated in the direction of the arrow. In this case, the 2nd driventransmission gear n2 is rotated at a higher speed than the 1st driventransmission gear n1.

Only the pin members 23 of the engaging means 20 corresponding to the1st driven transmission gear n1 are engaged in the cam grooves 21 v and22 v of the first and second cam rods 21 and 22, so that the swing pawlmembers 24 and 25 of this engaging means 20 are caused to project theirengaging pawl parts 24 c and 25 c radially outwards, and the engagingprojections 31 of the 1st driven transmission gear n1 being rotated areengaged with the engaging claw parts 25 c of the swing pawl members 25(see FIG. 16( c)), whereby the counter gear shaft 12 is rotated togetherwith the 1st driven transmission gear n1 at the same rotational speed asthat of the 1st driven transmission gear n1.

In FIGS. 16 to 24, the swing pawl members 24 and 25 and the engagingprojections 31 which are effectively transmitting motive power arecross-hatched.

In this 1st gear speed condition, the pin members 23 of the engagingmeans 20 corresponding to the 2nd driven transmission gear n2 have movedout of the cam grooves 21 v and 22 v of the first and second cam rods 21and 22 to project outwards, and the swing pawl members 24 and 25 of thisengaging means 20 are caused to retreat their engaging claw parts 24 cand 25 c inwards, so that the 2nd driven transmission gear n2 is in idlerotation.

The other, 3rd, 4th and 5th driven transmission gears n3, n4, n5 aresimilarly in idle rotation (see FIG. 16( b)).

When the gear shift driving motor 60 is operated to make a gear shiftinto the 2nd gear speed and the control rod operating element 70 ismoved rightward in the axial direction, the control rod operatingelement 70 operates to move the first and second cam rods 21 and 22rightward in the axial direction through the springs 52 s and 53 s ofthe lost motion mechanisms 52 and 53.

Referring to FIG. 17, with regard to the first cam rod 21, the swingpawl members 24 operated through the pin members 23 are not inengagement with the engaging projections 31 of the 1st driventransmission gear n1, and the first cam rods 21 are therefore moved withlittle resistance, so as to cause the pin members 23 having been engagedin the cam grooves 21 v to move out of the cam grooves 21 v and projectoutwards, whereby the swing pawl members 24 are swung and the engagingclaw parts 24 c are retreated inwards (see FIG. 17( c)).

With regard to the second cam rods 22 on the other hand, the swing pawlmembers 25 operated through the pin members 23 are in engagement withthe engaging projections 31 of the 1st driven transmission gear n1 andare receiving motive power from the 1st driven transmission gear n1, sothat a considerably great frictional resistance is exerted in swingingthe swing pawl members 25 for disengaging the latter. Therefore, even ifthe second cam rods 22 are moved by the force of the spring 53 s of thelost motion mechanism 53 to urge the pin members 23 to project outwardsalong the inclined side surfaces of the cam grooves 22 v, it isimpossible to push up and swing the swing pawl members 25. As a result,the cam rods 22 are stopped in a state in which the pin members 23 areabout to move outwards along the inclined side surfaces of the camgrooves 22 v, and the engagement is kept unreleased (see FIG. 17( c)).

With regard to the 2nd driven transmission gear n2, in the conditionshown in FIG. 17, the pin members 23 are not engaged in the cam grooves21 v in the first cam rods 21, so that the swing pawl members 25 show nochanges, and the swing pawl members 24 also show no change because thesecond cam rods 22 are at rest (see FIG. 17( b)).

When the control rod operating element 70 is moved further rightwardsunder the condition where the second cam rods 22 are at rest, the firstcam rods 21 are also moved rightwards. As a result, referring to FIG.18, with regard to the 1st driven transmission gear n1, the pin members23 move out of the cam grooves 21 v of the first cam rods 21 and projectoutwards so as to swing the swing pawl members 24 to retreat theengaging claw parts 24 c fully to the inner side (see FIG. 18( c)). Withregard to the 2nd driven transmission gear n2, the pin members 23 enterthe cam grooves 21 v of the first cam rods 21, and the swing pawlmembers 25 are swung by the urging forces of the torsion coil springs 27and the centrifugal forces of their engaging claw parts 25 c, to projectthe engaging claw parts 25 c outwards (see FIG. 18( b)).

Then, the engaging projections 31 of the 2nd driven transmission gearn2, which are being rotated at a higher speed than the counter gearshaft 12 rotated together with the 1st driven transmission gear n1,catch up with and abut on the outwardly projected engaging claw parts 25c of the swing pawl members 25 (see FIG. 19( b)).

In this instance, referring to FIGS. 19( b) and 19(c), there is a momentthat the engaging projections 31 of the 2nd driven transmission gear n2and the engaging projections 31 of the 1st driven transmission gear n2are simultaneously abutted on the engaging claw parts 25 c and 25 c ofthe respective swing pawl members 25 and 25.

Therefore, immediately after this moment, the counter gear shaft 12 iscaused to start rotation at the same rotational speed as that of the 2nddriven transmission gear n2, by the 2nd driven transmission gear n2rotating at a higher speed (see FIG. 20( b)), while the engaging clawparts 25 c of the swing pawl members 25 are separated from the engagingprojections 31 of the 1st driven transmission gear n1 (see FIG. 20( c)),whereby a practical upshift from the 1st gear speed to the 2nd gearspeed is carried out.

While the movement of the control rod operating element 70 ends at thispoint, the gear shift operation continues further. With the engagingclaw parts 25 c of the swing pawl members 25 separated from the engagingprojections 31 of the 1st driven transmission gear n1, the frictionalresistance fixing the swing pawl members 25 is eliminated. Consequently,the second cam rods 22 having been urged by the spring 53 s of the lostmotion mechanism 53 are moved rightwards, and the pin members 23 havingbeen engaged with the cam grooves 22 v begin to move out of the camgrooves 22 v to swing the swing pawl members 25, thereby retreating theengaging claw parts 25 c inwards (see FIG. 20( c)).

Then, with the second cam rods 22 moved further rightwards, the pinmembers 23 for the 1st driven transmission gear n1 move out of the camgrooves 22 v, so that the engaging claw parts 25 c of the swing pawlmembers 25 are completely retreated to the inner side (see FIG. 21( c)).Further, with regard to the 2nd driven transmission gear n2, the pinmembers 23 enter the cam groove 22 v, and the swing pawl members 24 areswung by the urging forces of the torsion coil springs 27 and thecentrifugal forces of their engaging claw parts 24 c, to project theengaging claw parts 24 c outwards (see FIG. 21( b)).

In this condition, the gear shift operation from the 1st gear speed tothe 2nd gear speed is completed.

The process of the upshift from the 1st gear speed condition to the 2ndgear speed condition, which is one step lower in reduction ratio thanthe 1st gear speed condition, is carried out as above. That is, in thecondition shown in FIG. 19 where the engaging projections 31 of the 1stdriven transmission gear n1 are in abutment on and in engagement withthe engaging claw parts 25 c of the swing pawl members 25 so that thecounter gear shaft 12 is being rotated at the same speed as the firstdriven transmission gear n1, the engaging projections 31 of the 2nddriven transmission gear n2 rotating at a higher speed catch up with andabut on the engaging claw parts 25 c of the swing pawl members 25,whereby the counter gear shaft 12 is rotated at a higher speed togetherwith the 2nd driven transmission gear n2, and the gear shift isachieved. Therefore, the engaging claw parts 25 c of the swing pawlmembers 25 are easily separated from the engaging projections 31 of the1st driven transmission gear n1, and the engagement is thereby releasedsmoothly. Accordingly, no force is needed to release the engagement, anda smooth operation and a smooth upshift can be achieved.

Similarly, an upshift from the 2nd to the 3rd gear speed, an upshiftfrom the 3rd to the 4th gear speed, and an upshift from the 4th to the5th gear speed can also be performed smoothly. Specifically, in theseupshifts, in the condition where one driven transmission gear (n) is inengagement with the swing pawl members 24, another driven transmissiongear (n) which is one step lower in reduction ratio than the one driventransmission gear (n) is engaged with the swing pawl members 24, to makethe upshift. Therefore, no force is needed to release the engagement,smooth operations are ensured, and there is no need for a clutch forgear shift. In addition, there is utterly no loss in changeover time atthe time of making an upshift, a loss of the driving force is avoided,the shift shock is small, and a smooth upshift can be achieved.

For example, in the 1st gear speed condition, as shown in FIG. 16( c),the swing pawl members 25 are in engagement with the engagingprojections 31 of the 1st driven transmission gear n1, and,simultaneously, the engaging claw parts 24 c of the swing pawl members24 are in an engageable state in the proximity of the engagingprojections 31.

Therefore, when due to vehicle deceleration a driving force is appliedfrom the rear wheel to the counter gear shaft 12 and the direction ofthe driving force is thus changed, the engagement of the engagingprojections 31 of the 1st driven transmission gear n1 is speedilyswitched from the engagement with the swing pawl members 25 to theengagement with the swing pawl members 24, whereby the engagement can besmoothly taken over and maintained.

Besides, in the case of the upshift from the 1st to the 2nd gear speed,the engaging claw parts 24 c of the swing pawl members 24 for the 1stdriven transmission gear n1 are retreated inwards from the engageablestate before the engaging projections 31 of the 2nd driven transmissiongear n2 are engaged with the swing pawl members 25. Therefore, theseswing pawl members 24 do not obstruct the gear shift (see FIG. 19( c)),and smooth and reliable engagement and disengagement are performed.

Now, the process of a downshift from the 2nd gear speed condition to the1st gear speed condition, which is one step higher in reduction ratiothan the 2nd gear speed condition at the time of vehicle deceleration,will be described below with reference to FIGS. 22 to 24.

FIG. 22 illustrates a condition immediately after deceleration from the2nd gear speed condition.

Due to the deceleration, a driving force is applied from the rear wheelto the counter gear shaft 12. Consequently, as shown in FIG. 22( b), theengaging claw parts 24 c of the swing pawl members 24 which have been inan engageable state move into engagement with the engaging projections31 of the 2nd driven transmission gear n2 lowered in rotational speed,so that the rotating power of the counter gear shaft 12 is transmittedto the 2nd driven transmission gear n2. In other words, the so-calledengine brake is being applied.

When the control rod operating element 70 is moved leftwards in theaxial direction by driving of the gear shift driving motor 60 for thepurpose of making a downshift to the 1st gear speed under thiscondition, the control rod operating element 70 urges the first andsecond cam rods 21 and 22 to move leftwards in the axial directionthrough the springs 52 s and 53 s of the lost motion mechanisms 52 and53. However, the second cam rods 22 are receiving motive power from the2nd driven transmission gear n2 because the swing pawl members 24operated through the pin members 23 are in engagement with the engagingprojections 31 of the 2nd driven transmission gear n2, so that there isa considerably large frictional resistance in swinging the swing pawlmembers 24 so as to disengage the latter. Therefore, the second cam rods22 are stopped at the moment that the pin members 23 are about to moveoutwards along the inclined side surfaces of the cam grooves 22 v, sothat the engagement of these swing pawl members 24 is left unreleased.

On the other hand, since the swing pawl members 25 operated through thepin members 23 are not in engagement with the engaging projections 31 ofthe 2nd driven transmission gear n2, the first cam rods 21 are movedleftwards with little resistance so that the pin members 23 having beenengaged with the cam grooves 21 v of the first cam rods 21 are moved outof the cam grooves 21 v and project outwards to swing the swing pawlmembers 25, thereby retreating the engaging claw parts 25 c inwards (seeFIG. 23( b)).

Besides, in relation to the 1st driven transmission gear n1, theleftward movement of the first cam rods 21 causes the pin members 23 toenter the cam grooves 21 v of the first cam rods 21, so that the swingpawl members 24 are swung by the urging forces of the torsion coilsprings 27 and by the centrifugal forces of their engaging claw parts 24c, so as to project the engaging claw parts 24 c outwards (see FIG. 23(c)).

Then, when the swing pawl members 24 are rotated together with thecounter gear shaft 12 to catch up with and abut on the engagingprojections 31 of the 1st driven transmission gear n1, there is a momentthat the engaging projections 31 of the 2nd driven transmission gear n2and the engaging projections 31 of the 1st driven transmission gear n1are simultaneously abutted on the engaging projections 24 c and 25 c ofthe respective swing pawl members 24 and 25, as shown in FIGS. 23( b)and 23(c).

Immediately after this moment and then on, the engagement with the 1stdriven transmission gear n1 rotated at a lower speed becomes effective,while the engagement with the 2nd driven transmission gear n2 isreleased, whereby the downshift from the 2nd to the 1st gear speed isperformed.

While the movement of the control rod operating element 70 ends at thispoint, the gear shift operation continues further. With the engagementof the engaging projections 31 of the 2nd driven transmission gear n2with the swing pawl members 24 released, the frictional resistancefixing the swing pawl members 24 is eliminated. As a result, the secondcam rods 22 having been urged by the spring 53 s of the lost motionmechanism 53 are moved leftwards, and the pin members 23 associated withthe 2nd driven transmission gear n2 and having been engaged with the camgroove 22 v move out of the cam grooves 22 v to swing the swing pawlmember 24, thereby retreating the engaging claw parts 24 c inwards (seeFIG. 24( b)). Besides, in the 1st driven transmission gear n1, the pinmembers 23 enter the cam grooves 22 v, and the swing pawl members 25 areswung by the urging forces of the torsion coil springs 27 and thecentrifugal forces of their engaging claw parts 25 c, to project theengaging claw parts 25 c outwards (see FIG. 24( c)).

In this condition, the gear shift operation from the 1st to the 2nd gearspeed is completed.

The process of the downshift from the 2nd gear speed condition to the1st gear speed condition, which is one step higher in reduction ratiothan the 2nd gear speed condition, includes the following operation.That is, in the condition where the engaging claw parts 24 c of theswing pawl members 24 are in abutment on and in engagement with theengaging projections 31 of the 2nd driven transmission gear n2, as shownin FIG. 23, the engaging claw parts 24 c of the swing pawl members 24catch up with and abut on the engaging projections 31 of the 1st driventransmission gear n1 rotating at a lower speed, whereby the engagementis changed over. Therefore, the engagement of the engaging projections31 of the 2nd driven transmission gear n2 with the engaging claw parts24 c of the swing pawl members 24 is released smoothly. Accordingly, noforce is needed to release the engagement, a smooth operation isensured, and a smooth downshift can be achieved.

A downshift from the 5th to the 4th gear speed, a downshift from the 4thto the 3rd gear speed, and a downshift from the 3rd to the 2nd gearspeed can also be performed smoothly. Specifically, in these downshifts,in the condition where one driven transmission gear (n) is in engagementwith the swing pawl members 24, another driven transmission gear (n)which is one step higher in reduction ratio than the one driventransmission gear (n) is engaged with the swing pawl members 24, to makethe downshift. Therefore, no force is needed to release the engagement,a smooth operation is ensured, and there is not need for a clutch forgear shift. In addition, there is utterly no loss in changeover time formaking a downshift, a loss of the driving force is avoided, the shiftshock is small, and a smooth downshift can be achieved.

For example, in the 2nd gear speed condition, as shown in FIG. 22( b),the swing pawl members 24 are in engagement with the engagingprojections 31 of the 2nd driven transmission gear n2, and,simultaneously, the engaging claw parts 25 c of the swing pawl members25 are in an engageable state in the proximity of the engagingprojections 31.

Therefore, when, due to vehicle acceleration, a driving force is appliedfrom the internal combustion engine to the 2nd driven transmission gearn2 and the direction of the driving force is changed, the engagement ofthe engaging projections 31 of the 2nd driven transmission gear n2 ispromptly switched from the engagement with the swing pawl members 24 tothe engagement with the swing pawl members 25, whereby the engagementcan be smoothly taken over and maintained.

Besides, in the case of the downshift from the 2nd to the 1st gearspeed, the engaging claw parts 25 c of the swing pawl members 25 for the2nd driven transmission gear n2 are retreated inwards from theengageable state before the swing pawl members 24 are engaged with theengaging projections 31 of the 1st driven transmission gear n1.Therefore, these swing pawl members 25 do not obstruct the gear shift(see FIG. 23( b)), and smooth and reliable engagement and disengagementare performed.

By axially moving the cam rods 21 and 22 which are in slidable contactwith the inner peripheral surface of the internal hole of the countergear shaft 12 so as to be movable in the axial direction and each ofwhich has the sliding contact surface provided with a plurality of camgrooves 21 v and 22 v at axially spaced locations, the pin members 23fitted in portions of the counter gear shaft 12 are advanced/retreatedto swing the swing pawl members 24 and 25, thereby causingengagement/disengagement of the swing claw members 24 and 25 with/fromthe engaging projections 31 in the driven transmission gears (n).Therefore, a gear shift can be achieved by switching over the engagementthrough advancing/retreating relevant pin members 23 with small amountsof movement of the cam rods 21 and 22. As a result, a structure can beadopted in which the spacings between adjacent ones of the driventransmission gears (n) rotatably supported on the counter gear shaft 12are reduced as shown in FIG. 1, whereby the size of the multi-stagetransmission 10 in the axial direction can be reduced.

At the time of acceleration by operating the internal combustion engine,even if an operation to make downshift is made by moving the control rod51 leftwards in the axial direction, this operation is not enough torelease the engagement between the swing pawl member 25 and the driventransmission gear (n) transmitting the motive power. For practicallymaking a downshift during acceleration, the frictional clutch 5 ismomentarily disengaged before the gear shifting operation, to causedeceleration, and, in this decelerated condition, the gear shiftingoperation is made to thereby cause smooth changeover to the engagementbetween the swing pawl member 25 and a driven transmission gear (n) onestep higher in reduction gear ratio than before. Thereafter, thefrictional clutch 5 is engaged, and acceleration is performed.

Incidentally, in a case where the frictional clutch 5 is not adopted, ameasure may be adopted in which the rotational speed of the driventransmission gear (n) is momentarily lowered by a drive source rotationdecelerating means such as ignition timing control means and fuelinjection amount control means, whereby downshift can be carried outsmoothly even during acceleration.

When an attempt to make an upshift is made by moving the control rod 51rightwards in the axial direction under a condition where a drivingforce is applied from the rear wheel to the counter gear shaft 12 due todeceleration of the vehicle, a shift shock will be generated uponengagement of the swing pawl member 25 with a driven transmission gear(n) one step lower in reduction gear ratio than before, whenacceleration is made subsequently. In view of this, a shifting-upoperation during deceleration is inhibited, whereby a shift shock can beprevented from being generated.

1. A multi-stage transmission wherein a drive gear shaft and a driven gear shaft are disposed in parallel; a plurality of drive gears and a plurality of driven gears are supported on the drive gear shaft and the driven gear shaft, respectively, in a manner that the drive gears and the driven gears are constantly meshed with each other to constitute meshing gear combinations for different speeds; one of the drive gears and the driven gears are fixed to the gear shaft supporting the same, while the gear shaft supporting the other of the drive gears and the driven gears is a hollow gear shaft; and the other gears are supported on the hollow gear shaft individually in an engageable and disengageable manner through engaging means, which are operated by gear shift driving means for gear shift: said engaging means includes: a first engaging device for causing a first gear of the other gears and the hollow gear shaft to abut with each other in a rotational direction of the gears to thereby cause the first gear and the hollow gear shaft to be rotated synchronously; and a second engaging device for causing a second gear, one step lower in reduction gear ratio than the first gear, of the other gears and the hollow gear shaft to abut with each other in said rotational direction of the gears to thereby cause the second gear and the hollow gear shaft to be rotated synchronously; wherein for making an upshift through changing over the engaging means from the engagement with the first gear to the engagement with the second gear by the gear shift driving means, the gear shift is made by engaging the second gear with the hollow gear shaft through the second engaging device in a state in which the first gear is in engagement with the hollow gear shaft through the first engaging device.
 2. The multi-stage transmission according to claim 1, wherein said engaging means further includes: a third engaging device for causing the first gear and the hollow gear shaft to abut with each other in a counter-rotational direction of the gears to thereby cause the first gear and the hollow gear shaft to be rotated synchronously; and a fourth engaging device for causing the second gear and the hollow gear shaft to abut with each other in said counter-rotational direction of the gears to thereby cause the second gear and the hollow gear shaft to be rotated synchronously; wherein for making a downshift by changing over the engaging means from the engagement with the second gear to the engagement with the first gear by the gear shift driving means under a driving force applied from a driven side, the gear shift is made by engaging the first gear with the hollow gear shaft through the third engaging device in a state in which the second gear is in engagement with the hollow gear shaft through the fourth engaging device.
 3. The multi-stage transmission according to claim 2, wherein arrangement is such that: when the first gear is in engagement with the hollow gear shaft by the first engaging device, the third engaging device is held in a state engageable with the first gear, and for making an upshift from the first gear to the second gear, the third engaging device is released from the state engageable with the first gear before the second gear is engaged by the second engaging device; and when the second gear is in engagement with the hollow gear shaft by the fourth engaging device, the second engaging device is held in a state engageable with the second gear, and at the time of making a downshift from the second gear to the first gear, the second engaging device is released from the state engageable with the second gear before the first gear is engaged by the third engaging device.
 4. The multi-stage transmission according to claim 1, wherein each of said engaging devices comprises: engaging projections provided at an inner peripheral surface of each of the gears at circumferentially spaced locations; a plurality of cam rods which are put in sliding contact with an inner peripheral surface of an internal hole of the hollow gear shaft so as to be slidable in an axial direction and which are provided in sliding contact surfaces thereof with a plurality of cam grooves at axially spaced locations; pin members which are fitted in through-holes radially penetrating portions of the hollow gear shaft and which are advanced and retracted while alternately making contact with the sliding contact surfaces of the cam rods and the cam grooves; and swing pawl members which are swingably supported in the hollow gear shaft and which are swung by advance/retreat of the pin members so as to be engaged with and disengaged from the engaging projections; the cam rods being movable by the gear shift driving means to thereby make a gear shift.
 5. The multi-stage transmission according to claim 2, wherein the transmission includes drive source rotation decelerating means for decelerating a drive source; and at the time of making a downshift under a driving force applied from a drive source, a downshift operation is conducted after deceleration by the drive source rotation decelerating means.
 6. The multi-stage transmission according to claim 5, wherein the drive source rotation decelerating means includes one selected from the group consisting of ignition timing control means, fuel injection amount control means, and motive power transmission control means by disengagement of a clutch.
 7. The multi-stage transmission according to claim 1, further comprising means for preventing an upshift in which the gear shift driving means changes over the engaging means from the engagement with the first gear to the engagement with the second gear, when a driving force is being applied from the driven side.
 8. The multi-stage transmission according to claim 2, wherein each of said engaging devices comprises: engaging projections provided at an inner peripheral surface of each of the gears at circumferentially spaced locations; a plurality of cam rods which are put in sliding contact with an inner peripheral surface of an internal hole of the hollow gear shaft so as to be slidable in an axial direction and which are provided in sliding contact surfaces thereof with a plurality of cam grooves at axially spaced locations; pin members which are fitted in through-holes radially penetrating portions of the hollow gear shaft and which are advanced and retracted while alternately making contact with the sliding contact surfaces of the cam rods and the cam grooves; and swing pawl members which are swingably supported in the hollow gear shaft and which are swung by advance/retreat of the pin members so as to be engaged with and disengaged from the engaging projections; the cam rods being movable by the gear shift driving means to thereby make a gear shift.
 9. The multi-stage transmission according to claim 3, wherein each of said engaging devices comprises: engaging projections provided at an inner peripheral surface of each of the gears at circumferentially spaced locations; a plurality of cam rods which are put in sliding contact with an inner peripheral surface of an internal hole of the hollow gear shaft so as to be slidable in an axial direction and which are provided in sliding contact surfaces thereof with a plurality of cam grooves at axially spaced locations; pin members which are fitted in through-holes radially penetrating portions of the hollow gear shaft and which are advanced and retracted while alternately making contact with the sliding contact surfaces of the cam rods and the cam grooves; and swing pawl members which are swingably supported in the hollow gear shaft and which are swung by advance/retreat of the pin members so as to be engaged with and disengaged from the engaging projections; the cam rods being movable by the gear shift driving means to thereby make a gear shift.
 10. The multi-stage transmission according to claim 2, further comprising means for preventing an upshift in which the gear shift driving means changes over the engaging means from the engagement with the first gear to the engagement with the second gear, when a driving force is being applied from the driven side.
 11. The multi-stage transmission according to claim 3, further comprising means for preventing an upshift in which the gear shift driving means changes over the engaging means from the engagement with the first gear to the engagement with the second gear, when a driving force is being applied from the driven side. 